霍邱李老庄BIF矿区磁铁矿-菱镁矿组合成因机制研究

Precambrian BIF magnetite and magnesite deposit at LiLaoZhuang is located in the central of Anhui Huoqiu iron ore belt of North China craton’s southern margin. The magnetite and magnesite output is unique at Huoqiu and the entire North China Precambrian region. The geologist are concerned about the forming mechanism of this particular mineral combination and the geological significance it represents. Especially the scientific problem of whether magnesite is primary sedimentary formed in local environment, or formed by later magnesia metasomatism needs to be solved urgently. Based on detailed field geological observations, this research plans to emphatically analyze the major and trace elements, C-O isotopic variation in magnetite, magnesite (iron magnesite) and dolomite, and determine Mg isotopic composition in magnesite and dolomite by utilizing EPMA, LA-ICP-MS, MC-ICP-MS and other experiment methods, in order to analyze the formation of magnesite deposit at LiLaoZhuang and its relation with siderite and dolomite, reveal the forming mechanism of magnesite. And finally try to investigate formation and evolution process of LiLaoZhuang deposits, and establish mineralization mechanism of magnetite – magnesite. The research project will provide reference for enriching Precambrian BIF ore type in Northern China and perfecting BIF metallogenic theory.

早前寒武纪李老庄BIF型磁铁矿-菱镁矿床位于华北克拉通南缘安徽霍邱铁矿带中部，该矿区内发育的磁铁矿-菱镁矿组合在霍邱及整个华北前寒武纪地区都是唯一的。这一特殊的矿物组合的成因、所代表的地质意义如何是地质学家所关心的，尤其是菱镁矿是局部环境下的原生沉积，还是后期镁质交代成因是急需解决的科学问题。本项目在详细野外地质观察的基础上拟采用EPMA、LA-ICP-MS、MC-ICP-MS等测试手段，重点分析磁铁矿、菱镁矿（铁菱镁矿）及白云岩的主微量元素组成、C-O同位素变化及菱镁矿和白云岩的Mg同位素组成，剖析李老庄矿床菱镁矿的形成与菱铁矿和白云岩的关系，揭示菱镁矿的成因，在此基础上探讨李老庄矿床的形成及后期演化过程，并建立磁铁矿-菱镁矿成矿机制。本项目的研究将为丰富华北前寒武纪BIF铁矿类型，完善BIF成矿理论提供借鉴。

华北克拉通南缘早前寒武纪李老庄BIF型铁矿-菱镁矿床罕见的同时发育BIF型铁矿体和菱镁矿体。矿床赋存于霍邱群吴集组云母石英片岩、变粒岩、中基性火山岩和镁质大理岩中，原岩为新太古代海相沉积的细碎屑岩-富镁碳酸盐岩系。矿体主要呈层状、似层状及透镜状产出，产状与围岩一致。铁矿石类型主要为硅酸盐型和碳酸盐型，少量石英型；矿石构造以条带状、条纹状和块状构造为主，少量浸染状，矿石富集重稀土元素，具有明显的La、Eu和Y的正异常，具有典型的BIF型铁建造特征，可以与Superior型BIF进行对比。菱镁矿为亮晶质白-灰白色，块状构造，菱镁矿中含较多Fe2+，说明菱镁矿并非由蒸发沉积而成，具明显的热液改造特征-变质重结晶和菱镁矿的次生加大，为典型的沉积-热液改造型菱镁矿。铁矿石的Sr/Ba值基本都大于1，具有明显的火山岩和海相沉积特征，Ti/V介于9.54~26.81之间，与火山建造较一致，表明成矿与海底火山作用有关。铁矿石与菱镁矿石均具有明显的Eu和Y正异常及较高的Y/Ho，暗示成矿物质主要来源于海水与海底热液的混合；二者Al2O3和TiO2含量较低且强烈亏损高场强元素，说明矿床的形成受陆源碎屑物质影响较小。.李老庄铁矿石及菱镁矿石均不具有Ce元素的负异常，且矿石中铁大都以Fe2+的形式存在，说明矿床形成于相对缺氧的偏中性热液还原环境。霍邱弧后盆地长轴方向近南北，呈现向西突起的构造形迹，靠近大陆边缘浅海地带，可能呈南北走向中部微向西凸的封闭或半封闭海湾，盆地中轴东侧即为李老庄矿床，菱镁矿的大量产出说明李老庄矿床形成于局限盆地或海湾环境，因此李老庄矿床形成环境为罕见的“盆中盆”-弧后盆地中的浅海海湾局限海盆。.李老庄矿床中发育罕见的磁铁矿-菱镁矿组合。对比与磁铁矿共生的菱镁矿单矿物、菱镁矿石及李老庄矿区围岩白云岩的C-O同位素组成，表明李老庄菱镁矿由富镁热液改造白云岩而成；三者C-O同位素组成的不同说明李老庄矿床在原始沉积物成岩以后，经历了区域低角闪岩相变质及后期热液作用的叠加改造。